1. Field of the Invention
The invention generally relates to image sensor systems; and in particular, the present invention relates to an image sensor including a memory interface.
2. Background of the Invention
Digital photography is one of the most exciting technologies that have emerged in the past years. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital cameras, for example, are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make digital cameras the hottest new category of consumer electronics products.
Digital cameras, however, do not work in the way that traditional film cameras do. In fact, they are more closely related to computer scanners, copiers, or fax machines. Most digital cameras use an image sensor or a photosensitive device, such as a charged-coupled device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS) device to sense a scene. The photosensitive device reacts to light reflected from the scene and can translate the strength of that reaction into electronic charging signals that are further digitized. By passing light through red, green, and blue filters, for example, the reaction can be gauged for each separate color spectrum. When the readings are combined and evaluated via software, the camera can determine the specific color of each segment of the picture. Because the image is actually a collection of numeric data, it can easily be downloaded into a computer and manipulated for more artistic effects.
In conventional digital imaging applications, an image sensor is coupled to an imaging processing unit (typically an integrated circuit or a chip) for receiving and processing the captured image. FIG. 1 illustrates a conventional digital imaging system including an image sensor coupled to an image processing unit. Referring to FIG. 1, image sensor 10, which can be a charged-coupled device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS) sensor, communicates with image processing unit 20 via a pixel data bus 12. After image sensor 10 captures an image, sensor readout is performed by exporting the pixel data one pixel at a time on the pixel-bit wide pixel data bus 12. Conventional image sensors provide either digital pixel data or analog pixel values as output signals on the pixel data bus. Here, the term “pixel data” is used collectively to refer to both the digital pixel data and the analog pixel value generated by an image sensor. Image processing unit 20 is coupled to a memory 24 for storing pixel data received from image sensor 10 before the image data can be processed or manipulated. Referring to FIG. 1, image processing unit 20 typically includes two interfaces: a sensor interface port 22a coupled to image sensor 10 for receiving sensor readout from the image sensor; and a memory interface port 22b coupled to memory 24 for storing the sensor readout. Thus, in operation, for every frame of image captured, image processing unit 20 first receives sensor readout from image sensor 10 one pixel at a time on pixel data bus 12. Image processing unit 20 directs the received pixel data for storage in memory 24. Thereafter, image processing unit 20 can access the frame of image data through memory interface port 22b. 
The conventional digital imaging system of FIG. 1 has several disadvantages. First, because pixel data are read out one pixel at a time from the image sensor, sensor readout can be undesirably slow, particularly for large image array. The speed of the conventional digital imaging system is limited by the pixel transmission rate of the pixel data bus 12 and the pixel data bus becomes the data transmission bottleneck of the imaging system. Second, image sensor 10 exports pixel data according to a preloaded pixel access pattern and the access pattern cannot be changed readily during sensor readout. Thus, if a change in the pixel access pattern is desired, image processing unit 20 has to first stop the current access and reload the new access pattern before sensor readout can begin again.
What is needed is an image sensor which provides a convenient and efficient pixel data interface with an image processing device.